Omicron "K" Down Belay Jacket

A lightweight, down-filled, technical belay jacket for climbing in raw cold & dry conditions. Designed especially for tough climbing days on classic mixed mountain routes and technical steep water-ice. Now also available in custom sizes.
Typical Operating Temperature
Guideline minimum temperature. Sleeping Bag TOT: you should get a night’s sleep, under normal circumstances. Clothing TOT: you should feel warm enough, assuming low/moderate activity. Sleep System TOT: relates to full system. For more details, see our Technical Briefing.
-10°C / 14°F
For clothing, weight is for size: M. For sleeping bags, weight changes with selected length/width. N.B. there may be some variation in the weights of finished products.
410g / 14oz
Omicron "K" Down Belay Jacket


  • A PHD K Series product: Ultimate warmth / weight ratio
  • PHD's Unique 1000 fillpower down
  • Water-resistant Ultrashell outer fabric
  • Ultralight 10X inner fabric
  • Stitch-through construction for lightness
  • Front closure - 2 way YKK Aquaguard water-resistant zip with bottom locking stud
  • 3 insulated pockets:
    • 2 zipped waist level pockets
    • 1 zipped chest pocket
  • Integral hood with wired peak, sized for a modern climber's helmet:
    • Adjustable front cord with ends tethered inside the hood
    • Wired peak
    • Volume reducer on the back of the hood
  • Reflector tape on front and back for night location
  • Clip loop to secure jacket at change-overs
  • Double exit adjustable hem cord
  • Cuffs: adjustable Velcro tab, to go over or under gloves
  • Drop-tail back
  • Stuff sac
  • Packed size: 13cm x 21cm*


*Please note: We make our stuff sacs big enough to allow fairly easy stuffing rather than reducing the jacket to the tightest minimum.



This K Series product has been created for one single reason, to take warmth-for-weight performance to the limit of what's possible. Unique 1000 down, super-light materials, and specific design features mean that the word Ultralight now applies to this wide range of gear that will take you to the Poles as well as on a Sahara Marathon. More on the K Series...

Also available in custom sizes. We make clothing to order, so if you’re not a standard size we can make it to fit. To order a non-standard custom size, select the Custom Sizing tab, choose your body width, and then the body/arm lengths, shorter or longer, that work for you. A custom size takes us a little longer (typically two weeks longer) to make and, because it’s personalized, it’s non-returnable and our customer loyalty scheme can’t be applied.

The Omicron “K” is a down filled superlight belay jacket which squares perfectly with the modern fast-and-light approach to technical water ice and mixed test pieces: a good fit, freedom of movement, minimum weight and maximum warmth are essential, but so is dependable protection on cold stances. The stitch through construction cuts excess weight, while allowing the 1000 fill power down to deliver a TOT* rating of -10°C.

Designed to work over or under a climbing harness, the minimally featured, trim fit makes for light movement while climbing. The fully adjustable helmet hood and high front zip provide maximum warmth, and lock right down in gnarly conditions. Minimalism doesn’t come at the expense of function however: this is a highly technical jacket, and the high-viz reflective strips for location, glove-friendly zip pulls and a clip loop to secure the jacket at change overs are all there because they need to be.

Although down filled Belay jackets are really in their element in cold, dry conditions, the water-resistant Ultrashell fabric and YKK Aquaguard zips help to protect against damp snow or dripping ice if the temperatures rise. If you’re venturing into cold and wet conditions though, take a look at our Primaloft filled Belay Jackets.

Please note: this jacket is designed and sized to be worn over the top of outer layers. It provides additional warmth on cold belays and can be swapped between climbers at pitch change-overs. If you want a trimmer, more athletic fit, select the next size down.

*TOT: Typical Operating Temperature

Outdoor photography: Paul Horsman.

Ultrashell (weight: 40 gsm)

One of the lightest of PHD's coated materials. This ripstop nylon provides complete wind block and good water resistance at minimal weight. Ultrashell gives an excellent upgrade to performance with very little weight penalty.

Ultralight 10X fabric (weight: 26 gsm)

10X, PHD's first 10 Denier Fabric. 9,000 metres of the yarn used to weave 10X weighs just 10g, that's about the same as a £1 coin! For all its thinness 10X is amazingly downproof and with reasonable care will go on giving you good service for years.

1000 fillpower down

This rarest of all downs we reserve for the K Series. European goose down of unique quality. 1000 provides the highest warmth-for-weight performance possible from down. More on our down insulation.

Works well with
From 191.64 150g
Primaloft-filled mountaineering mitts. Confidence and warmth in cold and wet conditions.
From 1,222.26 770g -15℃
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